"The Natural Philosopher" wrote in message
...
robgraham wrote:
Why is that when you are at one's intellectual worst that the more
complex of discussions occur - solution of the world's problems,
etc !!!

This particular one revolved round ac and dc motors - covering single
phase v. 3 ph induction motors and their relative power /torque
capabilities and how they work, which was hard work 45 years after
university, but what triggered the whole discussion was how Bosch can
claim that a lawnmower with a 36v Li-polymer powered dc motor can have
the same capability as a 1700w ac motor. OK 1700w is a comfortable
2hp but single phase induction motors are not too good on torque, and
if I remember that is where dc motors score but are modern ones going
to match a 1700w ac motor ?

Can anyone help or point me at a site on this topic please.

certainly 36v and about 50A is not unusual in the largest 'DC' model
aircraft motors. Mind you there is no such thing as a DC motor.

They are all 3 phase. - a DC brushed motor simply uses a commutator to
generate the AC..

??? In a DC motor the field is DC. The commutator is to ensure the field
in the rotor is aligned wrt the stator to provide continuous torque. Not
sure where you get 3 phase from. From a standard single pole pair DC motor
I would call it "single phase". 2 pole pairs on a DC motor is common place
for starter motors. I haven't come across with 3 pole pairs with 3 sets of
brushes?


If you are sticking 1700W in, its got to come out as heat, noise or as
shaft power.

I remain unconvinced about power in and useable power out for mains motors.
At present I would have hoped green thinking would have had some meaningful
measure of real power out, not just turned into heat. There is still the
concept that the more power in the better the motor! Generally small
induction motors are very inefficient, DC motors are inherently very
efficient. AC (brushed) motors are less efficient than DC and the best
thing that could be done would be to rectify the mains first.


I doubt efficiencies are any less on a brushed PM versus induction motor,
..


Rob